This invention relates to the doping of crystalline substrates.
The doping of crystalline substrates such as silicon carbide, diamond and other such materials to provide such substrates with semi-conducting properties and/or optically active centres, is well established in the art. Dopant atoms can be introduced into the substrate by using ion implantation techniques.
European Patent Publication No. 0 209 257 discloses a method of producing a doped diamond which includes the steps of bombarding the diamond with ions of a suitable energy to produce a damaged layer of point defects in the form of vacancies and interstitials within the crystal lattice, the bombardment being carried out at a temperature sufficiently low to inhibit diffusion of the point defects, followed by annealing the damaged substrate. Dopant atoms are introduced into the damaged layer by ion implantation during, before or after the damage-creating bombardment. The dopant atom implantation also takes place at a temperature sufficiently low to inhibit diffusion of the point defects in the damaged layer.
European Patent Publication No. 0 573 312 discloses a method of producing a doped diamond which includes the steps of creating a damaged layer of point defects in the form of vacancies and interstitial atoms within the crystal lattice of the diamond using low dose ion implantation at low temperature, introducing dopant atoms into the damaged layer using low dose ion implantation at low temperature, rapidly annealing the product to reduce lattice damage and to cause dopant interstitial atoms to diffuse into lattice positions, and repeating the doping and rapid annealing steps until a doped diamond having a desired amount of dopant is produced.
In the methods described above, the implanting of the diamond substrate is at a temperature low enough to "freeze" the implanted atoms and intrinsic point defects (vacancies and interstitials) which are created during the implantation process, such that they cannot diffuse out of the implanted layer being created. The temperature required for diamond is preferably well below room temperature and typically liquid nitrogen temperatures. In order to get a good intermix of implanted atoms, vacancies and self-interstitials, and a large width for the implanted layer, the ions are usually implanted at a series of different energies and doses at these energies. This cold implantation step is then followed by annealing (usually rapid annealing) at a suitable temperature. The implanted atoms which combine with vacancies then become activated dopant atoms, and the self-interstitials, which similarly combine with vacancies, reduce the implantation damage.
The width of the implanted layer, and vacancy density in it, are important to this method because some of the implanted dopant atoms and self-interstitials can escape and diffuse away before recombining with the vacancies in the layer, thus leaving behind a residue of these vacancies.
Unfortunately, vacancies in diamond are optically and electrically active and have thus, in most cases, a deleterious effect on the required properties of the doped layer. Even if they were not optically or electrically active, their presence leads to scattering of charge carriers which is undesirable in high quality doped materials. Although further annealing at higher temperatures usually reduce these undesired effects, it has not been possible to remove them altogether by, for example, enticing all the vacancies to diffuse out of the layer. This is prevented by the tendency of the vacancies to agglomerate and form more complex defects which, like the single vacancies, are usually deleterious. Thus, it is desirable to prevent the interstitial atoms (both dopant and self) diffusing away before recombining with vacancies. Obviously, the wider the implanted layer and the more vacancies it contains, the lower the fraction of interstitials (both dopant and self) which can escape. However, to increase the vacancy density requires larger ion doses which, in turn, also create more interstitials, and although a larger fraction of these interstitials now combine with vacancies, it has been found that the total amount of interstitials escaping increases, which, in this way, leaves behind an even larger density of vacancies in the implanted layer. If, on the other hand, very low ion doses are used to diminish the total density of residual vacancies, the fraction of implanted dopant interstitials which can escape, increases, and too few activated dopant atoms may remain.